1. Field
Embodiments of the present invention relate to image sensors and, in particular, to auto-focus in image sensors.
2. Discussion of Related Art
In general, digital imaging devices may be one of two types: automatic focus (auto-focus) and fixed-focus. The fixed-focus devices usually are incapable of affecting lens position or changing the aperture and, instead, rely on a large depth of field where the object appears to be in focus. Although acceptable in many cases, the images captured by fixed-focus devices are not as sharp as those captured by auto-focus devices.
There are different methods and associated apparatus to automatically focus a digital imaging device. In one method the lens is moved in a direction that increases the intensity of the light on light sensing elements and is stopped after the maximum intensity is achieved. Another automatic focusing technique relies upon finite impulse response (FIR) filters to determine the edge features and their sharpness. Such a technique is less effective in case of images with widely varying intensity/color values, since averaging distorts results.
There are many conventional auto-focusing techniques for image sensors, including measuring the distance from the object of interest and mapping the distance to some measurable quantity that drives the optical system to arrive at the correct focus. This method of auto-focusing is performed in parallel to the main imaging path, which is ideal for film-based cameras.
As digital imaging devices become dominant in the market, auto-focusing techniques rely mainly on the data obtained from the main imaging data path. The basic assumption is that the best focus condition is achieved when the image contains the maximum amount of high frequency information, measured by applying digital filtering to a portion of the digitized image data. The computed energy of the filtered spectrum is used as a measure of frequency content.
Conventional image sensors work well, but can be improved upon. This is true for auto-focus capabilities as well as other capabilities. For example, image sensors that provide preview capabilities where an image is scaled down to the format of small screens so that a user can view the image prior to taking a picture also could be improved upon.